1. Field of the Invention
The present invention relates to a ridge waveguide semiconductor laser diode, and particularly, to a ridge waveguide structure effective for improving reliability after mounting a ridge waveguide semiconductor laser diode.
2. Description of the Related Art
A semiconductor laser diode has recently been widely used for a light source for an optical disk such as a DVD, and a light source for medical equipment, processing equipment, optical fiber communication and the like. Specifically, a semiconductor laser element comprising a nitride semiconductor (such as an InxAlyGa1−x−yN semiconductor, where 0≦x, 0≦y, and x+y≦1) attracts attention as a semiconductor laser element capable of emitting light in the range from relatively short-wavelength ultraviolet to red.
With regard to the stripe structure for controlling a transverse oscillation mode, the ridge waveguide structure is frequently employed in such semiconductor laser diodes, because of the capability to realize gain guiding and index guiding simultaneously. Also, since a nitride semiconductor laser diode is frequently constructed on a sapphire substrate whose thermal conductivity is low, it is advantageous to mount the nitride semiconductor laser diode by a face-down bonding (junction-down) method where the p-side electrode, which is close to the active layer, is placed downward to increase the heat release from the laser element.
In addition, even in the case where a nitride semiconductor substrate other than a foreign substrate such as a sapphire substrate is used, it is advantageous to use a submount having a larger thermal conductivity than the substrate, or to mount the p-type layer side down by the face-down bonding method in order to inhibit deterioration of the semiconductor element due to heat generation by its high power output.
FIG. 5 is a schematic cross sectional view showing a ridge waveguide laser diode attached by the face-down bonding method according to Japanese Unexamined Patent Publication No. 2000-58965. The nitride semiconductor laser diode 10 shown in FIG. 5 is bonded face-down on the mounting substrate 20 by a conductive joining material such as solder 23. The nitride semiconductor laser diode 10 is a ridge waveguide laser having a layer structure. In this structure, an n-type nitride semiconductor layer 12, an active layer 13, and a p-type nitride semiconductor layer 14 are sequentially formed on the substrate 11, and a stripe ridge 14a is constructed in the p-type nitride semiconductor layer. In the nitride semiconductor laser diode 10, the n-side electrode 16, the p-side electrodes 15 and 19 are bonded to the electrodes 21 and 22 on the mounting substrate 20 by a conductive joining material such as solder 23.
However, the conventional ridge waveguide semiconductor laser diodes described above have problems such as defects in the initial laser quality that occur after bonding the laser diode 10 on the mounting substrate 20, or deterioration in their lifetime.